Typically, a switching control circuit may be connected between a direct current power supply and a power load in series to obtain controlled activation and deactivation of the power load by an external trigger signal. When the impedance of the power load has a substantial inductive component, a free wheeling element, e.g. a flyback diode, may be placed in parallel to the power load to provide a path to conduct transient currents which are induced when altering the supplied voltage to the inductive load.
A switching control circuit commonly comprises a semiconductor switch, for example a metal-oxide semiconductor field-effect transistor (MOSFET). In such MOSFET's, a voltage on the gate contact determines conduction of a channel between the other two contacts, called source and drain. If a sufficient voltage over the gate is maintained, a supply voltage will be supplied to a load terminal, and hence also to the flyback diode. If the gate voltage is reduced in order to deactivate the load, the internal resistance of the transistor, i.e. over the source-drain channel, will rise. However, the inductive component of the load will resist a change in its power supply and will maintain the current flowing through the transistor at a high level. The increased resistance of the transistor will therefore generate a substantial amount of heat, and may result in damage to the circuit if the gate voltage is reduced too fast with respect to the time needed for the flyback diode to divert the current flow.
In the German patent publication DE 4013997 A1, a switching control circuit for controlling a DC current through an inductive load is disclosed which comprises a measuring means and evaluating means for maintaining the gate voltage sufficiently high while deactivating the load until activation of the flyback diode is detected. Particularly, the measuring means measures a voltage at the semiconductor terminal connected to the load and to the flyback diode. When the semiconductor switch is turned off, the evaluating means maintains the MOSFET gate current at a high value until the load terminal voltage reaches an upper threshold value indicating that the flyback diode is contributing to the current flow, after which the gate current is further lowered. Similarly, at the activation of the semiconductor switch, the gate current is kept at a low value until the measured voltage exceeds a lower threshold value, indicating that the flyback diode stopped conduction, after which the gate current is increased.
This arrangement may reduce the undesirable effects of power dissipation in the semiconductor, but because the gate current is not reduced before the flyback diode starts to conduct, deep voltage transients at the deactivation of the switch might occur, which can lead to increased electromagnetic emissions. Such emission may be avoided by reducing the initial discharge current to a lower level, which in turn leads to longer turn-off times and consequently increased overall power dissipation in the semiconductor switch.
Furthermore, because the power dissipation of the semiconductor switch is proportional to the drain current times the drain-source voltage, it is the highest during the reverse recovery time, which, for high performance semiconductor switches with integrated anti parallel diodes, e.g. IPD50N04S4-10, may be in the range of 30 ns. The typical delay of fast comparators (e.g. LM119) with high input voltage capability required for the full voltage swing at the load terminal is typically in the range of 80 ns. Therefore the comparator delay is significantly increasing the power dissipation within the semiconductor switch because the increase of the charging current is delayed.
In the United States patent publication U.S. Pat. No. 5,801,458, another method for controlling a DC current through an inductive load is disclosed, in which the current for turning off the semiconductor switch is also reduced from a higher initial value to a value so that the semiconductor switch is only completely turned off when the free wheeling element has started conduction. In this publication, a turn-off discharge current source is used, which is continuously reducing the discharge current in dependency of the voltage on the free wheeling element so that minimum current is reached when the voltage on the free wheeling element is reaching 0V.
Although this method can reduce the discharge current to a low value when the free wheeling element starts to conduct, the method can only start to reduce the discharge current after the voltage on the flyback diode has started to drop, which may be too late to react for the current source to reduce its current, especially taking an additional delay of the measurement circuit into consideration. This may lead to a rapid voltage change on the free wheeling element and may cause increased electromagnetic emissions. In order to avoid such emissions, the initial discharge current must be reduced to a lower level, which in turn leads to longer turn-off times and consequently increased overall power dissipation in the semiconductor switch.